As a Senior Scientist at NXP Semiconductors

I currently work in the Advanced Devices and Technologies group (and before that, the Device Modelling and Characterisation group) within the Research and Development sector at NXP Semiconductors.

Device Physics and TCAD Simulation

My work mainly involves performing computer simulations of semiconductor devices using TCAD software, in order to determine and understand the internal mechanisms behind the operation of these devices (in terms of voltages, electric fields, current flow, etc.) and to make predictions of the performance of the devices in different operation regimes (e.g., at high voltages or current levels).

Compact Modelling

Much of my experience is in the development of computational models for describing the (electrical and thermal) behaviour of high-voltage semiconductor devices, such as LDMOS transistors and GaN HEMTs. These (so-called “compact”) transistor models are then used in circuit simulations in order to predict the behaviour of the circuit and to help in the optimisation of the circuit design.

Device Characterisation

In support of the simulation and modelling activities, I also spend some time in the measurement lab performing on-wafer measurements of the DC and AC characteristics of high-voltage devices. The measurement data can then act as a target for the models and simulations, or as a test for the predictions of device performance.

As a Research Fellow in Physics at the University of Exeter

My research was primarily concerned with the electronic properties of diamond, with respect to its viability as a material for use in future semiconductor devices. Diamond's extreme material properties make it attractive for use in devices that would be subject to conditions under which traditional semiconductor materials (such as silicon or germanium) might fail. In my research, I have worked on several topics, including the following:

Oxygenation and Hydrogenation of Diamond Surfaces

Hydrogen termination on the diamond surface is known to lead to a negative electron affinity and a low ionisation potential. Both of these effects encourage electron emission from diamond, which makes hydrogenated diamond suitable for use in devices such as cold cathodes, and surface-conductive devices that exploit the transfer-doping effect. However, oxygen termination leads to a positive electron affinity and a very large ionisation potential, severely hindering electron emission. For these reasons, the interaction of hydrogen and oxygen on the diamond surface is of much interest to those interested in the relevant device applications.

Boron-Nitride:Diamond (111) and (100) Interfaces

Boron nitride is isoelectronic with carbon, and indeed its allotropes have many properties similar to those of carbon, including similar crystal structures (e.g., graphite-like and diamond-like). Its diamond-like form (cubic boron nitride) is — like diamond — extremely hard, has good thermal conductivity, and has a large, indirect electronic bandgap. Interfaces of diamond and boron nitride make for interesting electronic junctions, and the effect of the small difference in lattice parameter between the two materials is also a matter of concern.

C60 and Fluorinated Fullerenes on Diamond Surfaces

Buckminsterfullerene (C60) has been identified as a potential transfer dopant for diamond. That is, molecules of C60 covering the diamond surface could extract electrons from the diamond due to their high electron affinity. This would leave behind a layer of electron-holes in the diamond substrate, which would lead to a high p-type conductivity across the diamond surface. Fluorinated fullerenes (such as C60F36) have even greater electron affinities, and therefore would be expected to extract electrons from diamond with greater efficiency. This could lead to the manufacture of diamond devices with tailor-made surface conductivity characteristics.

Substitutional B–P Pairs in Diamond

Boron is a near-ubiquitous impurity in diamond, while phosphorus is deliberately introduced to act as an electron donor for the creation of n-type material. Boron is known to act as a shallow electron acceptor, and hence could compromise the electrical activity of phosphorus-doped diamond. The interaction between boron and phosphorus defects in diamond, and the resulting electronic properties, are of much interest with regard to the performance of devices such as p-n junctions.

Aluminium-Nitride:Diamond (111) Interfaces

Aluminium nitride is another wide-bandgap material that could form electrical junctions with diamond having useful properties. Its lattice parameter is much greater than that of diamond, although it may be possible to find arrangements of aluminium nitride on diamond where there exist coincidences of multiple lattice parameters. In any case, the growth of aluminium nitride on diamond, the structure of the interface, and the electrical properties across the junction are interesting areas of study. Hetereojunctions of n-type aluminium nitride and p-type diamond could make for bipolar p-n junctions with good diode characteristics.

Electrical (Donor) Levels of Chalcogen (S, Se, Te) and Pnictogen (N, P, As, Sb) Substitutional Defects and Their Hydrogen Complexes in Diamond

Presently, phosphorus is the most effective electron donor impurity for creating n-type diamond in practice. However, it is not as efficient an electron donor as boron is an electron acceptor. As a result, the search for a better donor in diamond continues. It is useful to predict the electronic behaviour of other, larger group-V impurities (such as As and Sb) in diamond in order to guide experimental investigations. Complexes of group-VI impurities (such as S, Se, and Te) with atoms of hydrogen can also be considered, as they may too behave as efficient electron donors in diamond. However, hydrogen could compromise the electrical activity of the aforementioned group-V impurities.

Aluminium:Diamond and Copper:Diamond (111) Interfaces

The structural stability and electronic properties of metal contacts on the diamond surface is naturally of interest to those wishing to fabricate electronic devices based on, say, the high p-type conductivity measured across diamond surfaces. Aluminium and copper are two metals that could form contacts with diamond with useful and differing properties.

Diffusion of a Carbon Dimer Along [110] Channels in Diamond

Although diamond is very tolerant to extreme conditions, damage caused through, say, ion implantation or heavy irradiation could seriously affect the performance of diamond-based electronic devices. The possibility of migration of the di-self-interstitial (or interstitial carbon dimer) through the diamond lattice has been studied in this work.